1. Field of the Invention
The present invention relates to a method for manufacturing a solid-state image sensor.
2. Description of the Related Art
The structure of a transfer path through which a charge is transferred from a charge accumulation region of a photodiode to a floating diffusion is of prime importance for a solid-state image sensor. If especially the transfer path immediately beneath the gate electrode of a transfer MOS transistor is not formed optimally, the charge accumulated in the charge accumulation region remains without being completely transferred to the floating diffusion, thus leading to generation of, for example, a residual image and noise.
Japanese Patent Laid-Open No. 11-274454 describes a technique of obliquely implanting arsenic into a semiconductor substrate using a photoresist and a control electrode as a mask material to form a bypass region. Japanese Patent Laid-Open No. 2000-012823 describes a technique of implanting impurity ions into a silicon substrate using a gate electrode as a mask, forming silicon oxide films on the two side surfaces of the gate electrode, and further implanting impurity ions into the silicon substrate, thereby forming an LDD (Lightly Doped Drain) portion and a diffusion region.
Unfortunately, a method of obliquely implanting ions into a semiconductor substrate, as described in Japanese Patent Laid-Open No. 11-274454, is incompatible with pixel miniaturization because the photoresist produces a greater shadowing effect upon implantation as pixel miniaturization progresses. On the other hand, in the method described in Japanese Patent Laid-Open No. 2000-012823, the resist mask produces no shadowing effect upon implantation. However, the method described in Japanese Patent Laid-Open No. 2000-012823 is intended to form an LDD structure, so the LDD portion and the diffusion region (accumulation region) adjacent to it inevitably have different impurity concentrations. Hence, in transferring a charge from the diffusion region (charge accumulation portion) to the floating diffusion via the LDD portion, the difference in impurity concentration between the LDD portion and the diffusion region may generate a potential barrier, which may hinder the charge from being completely transferred.